Add support for aliases in locations

An alias can be specified for a location using alias_root instead of
www_root.  Aliases in nginx are similar to root directives except that
the part of the request that matches the location is stripped when
creating the path to file.  See
http://wiki.nginx.org/HttpCoreModule#alias for an example.
Merge branch 'location-alias' of https://github.com/cwarden/puppetlabs-nginx

Conflicts:
	manifests/resource/location.pp
	templates/vhost/vhost_location_alias.erb